1. Field of the Invention
The present invention relates to an FM tuner with an RDS (Radio Data System) demodulator in which a tuning is decided from a count of an intermediate frequency. More particularly, the invention relates to an FM tuner which decides a tuning from a tuning signal (hereinafter called an SD signal) in an alternative frequency search (hereinafter called an AF search) and in a program type search (hereinafter called a PTY search).
2. Related Background Art
The present applicant has proposed an FM tuner in which instead of the SD signal generated in the tuner, the value of an intermediate frequency counted for a predetermined period is used as a parameter for the decision of a tuning and the tuning operation is continuously controlled until a tuning is decided. Specifically, as shown in FIG. 4, during the tuning operation, the receiving frequency is raised, and the intermediate frequency is counted for a predetermined period, e.g., for 80 nsec if the SD signal has a low potential, i.e., if a tuning is indicated, and the tuning is finally decided if the checked count value of the intermediate frequency corresponds to the value of the intermediate frequency of 10.7 MHz .+-.15 kHz. If the SD signal has a high potential or if the checked count value does not correspond to the value of the intermediate frequency of 10.7 MHz .+-.15 kHz, the receiving frequency is again raised.
By counting the intermediate frequency to decide a tuning and control the tuning operation, it becomes possible to prevent an erroneous judgement of a tuning decided by the SD signal because of noises during the tuning operation, and to provide a stable and highly precise tuning control in real time and always at the maximum sensitivity.
However, in the case of an AF search or PTY search by a conventional FM tuner with an RDS demodulator which decides a tuning from the count value, it takes a long time to perform the AF search or PTY search.